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This project has not received attention for a long time. I plan to have pictures and a schematic, soon. I'm an adult with adult responsibilities so I rarely get to my hobby projects. I'm also nearing the end of my degree program so I've been putting a lot of time into my studies.

I finished wiring up the test situation this week. Although they will not persist in the final build, I used the tiny Radio Shack transformers for testing. They don't sound great. I have considered buying some Edcor small signal transformers, although they are expensive and will have a long lead time so I'm conflicted.

Another thing I realized after the build: the way 12J8 tubes function is based on grid leaking. Basically, a small portion of electrons strike grid #1 and leak off through the grid resistor. In my circuit, the volume potentiometer (50k) is not grounded but connects to a bypassed 1.5 M resistor. This results in the small current developing a voltage of -0.9v on the grid, which sets the operating point somewhere that the incoming signal won't drive the grid positive.

Except, when I hook the music player up to the input, it drains this voltage away. Not good. I need to install a capacitor between the input jack and the volume control because this voltage needs to be present for the tube to be properly biased. If not, the grid being driven positive will cause distortion especially at lower volumes.

I think it will be a fine project once I get some kinks worked out. I've thought of trying 12.6v AC transformers for the outputs and charting their response. I think some would be surprised. Remember that we are dealing with 20 milliwatts of power here. Plenty for headphones.

For anybody who wants to do some reading, here's a link to infrared radiation. Maybe it will help shed some light on why I'm going to paint my enclosure black after I get finished drilling ventilation holes.

Thats what you said in the url.That right there says that your using the 12j8 for voltage gain then power transitor to output the speakers

No, it doesn't.

What I'm saying is the 12J8 was originally designed for car radio use, where it would do what you are saying. This was very common in 1960s car radios before transistors were able to do things like amplify RF or show high impedances well. So, the 12J8 (and other power tetrodes like it) drove the base of the power transistor in the car radio so it could make 1-2 watts of power for a speaker. That's not what I'm doing here.

Original design use. Like the 6L6 was designed for audio amplification. This hasn't stopped some people from using them as radio transmitters or series pass tubes in DC supplies.

My headphone amp is just one 12J8 tube per channel, making a few milliwatts for headphones which is more than enough. There is no power transistor in my headphone amp. There aren't even any semiconductors, until I install the pilot LED. If I could rig a lens to bend orange light from the tubes to a hole in the front, I wouldn't even need an LED. I know it's common to build tube/MOSFET hybrids to drive headphones without output transformers (I made one before). In this case, I decided to leave it out and use the 12J8 as they are designed to be used, driving a plate transformer for a little bit of power.

I turned a knob from walnut today for the volume control, and sealed it by rubbing clear wax into it. I still haven't picked out an LED.

The 20 milliwatts from this is plenty loud. If I wanted to go for power, I'd get a couple of power transistors and drive them with an op amp. This project was about the novelty of taking a low-power tube and seeing how it would perform. The 12J8 is a pretty obscure tube. If I wanted to take it one step further, I could use the diodes to make a peak detector and drive a 50 uA panel meter.

As far as headphone amps go, this will probably be my last real effort. I have a lot of other things going on and I'm more interested in radio circuits these days. I have a pair of 2000 ohm magnetic headphones, and those are meant to be driven directly from the plate of a triode. Piano sounds really good through them but voice is kind of blah.

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